ANCHORAGE, Alaska — Before serial killer Israel Keyes murdered an 18-year-old barista in Alaska, he also targeted the young woman’s boyfriend before changing his mind, according to the lead investigator in the slaying.

Homicide Detective Monique Doll said the plan was part of Keyes’ escalation of his killing from single victims to pairs, likening it to a drug addict’s need to continue to take more drugs to match the feeling of the initial high.

Shortly before killing Bill and Lorraine Currier in Vermont in 2011, Keyes came close to killing a couple parked in a car at an Anchorage park, police said. He also considered shooting the police officer who came to shoo the pair away from the closed park, but abandoned his plan when a backup officer arrived.

Keyes, who recently killed himself in his jail cell, abducted Samantha Koenig on Feb. 1 at the Anchorage coffee stand where she worked after he climbed in the drive-through window. He bound Koenig with zip ties, then waited with his victim on the floor of the darkened structure, said Doll.

Doll said Tuesday that investigators reviewing the abduction caught by security cameras initially thought Keyes had Koenig on the floor to wait out anyone who might have seen him climb through the window or to wait for things to calm down.

“What we found when we interviewed Mr. Keyes was that he knew that the coffee kiosk closed at 8 p.m. and he knew that because Samantha didn’t have a vehicle there, somebody was going to come and pick her up,” Doll told The Associated Press. “He was waiting for her ride to come there because he intended to take that person as well.”

Doll said she doesn’t know why Keyes changed his mind about also killing the boyfriend, as he had targeted the Curriers.

“I don’t know why he left, but he made it very clear that, you know, that was part of the plan,” she said. “It was very fortuitous for Mr. Tortolani not to have run into Mr. Keyes.”

Keyes and Tortolani would have a later encounter.

Early the next morning, about 3 a.m., Tortolani saw someone in a mask standing near his truck. He confronted the person, whom Doll said was Keyes trying to steal a debit card from the pickup. Tortolani ran inside the home he shared with Koenig and her father to get his help.

Keyes, who had parked a distance from the Koenig home, was gone by the time the men came out, Doll said. Also gone was the debit card for an account Koenig shared with Tortolani.

There is no local phone listing for Tortolani, and he did not immediately return a phone message seeking comment Tuesday left with his employer.

Keyes, who confessed to killing Koenig and at least seven others across the country, told investigators he raped and strangled Koenig, leaving her body in a shed for two weeks while he went on a cruise.

Keyes was arrested in Lufkin, Texas, in March after using the boyfriend’s stolen debit card. Keyes was found dead earlier this month after slitting his wrist and strangling himself with a rolled-up sheet in his Anchorage jail cell, where he was awaiting trial next year for Koenig’s slaying.

Three weeks after Keyes was arrested, Koenig’s remains were found in a frozen lake north of Anchorage.

Before his death, Keyes also confessed to four murders in Washington state, two in Vermont and one on the East Coast with the body disposed of in New York over the past decade. There also could be three additional victims, for a total of 11 murders, according to the FBI.